1. Field of the Invention
The present invention relates to color image forming apparatuses, and more particularly to improvement of a color image forming apparatus where charging, exposure and development are repeated plural times on an image carrier thereby a plurality of toner images are formed in superposing and then transferred collectively to an image receptor.
2. Prior Art
In the prior art, various image forming apparatuses are proposed where after charging, exposure and development are repeated plural times and a plurality of toner images being different in color are formed on an image carrier, these toner images are transferred collectively to a recording paper thereby color images are obtained (e.g., JPA No. 76766/1985).
As a charging device used in such a color image forming apparatus, for example, that shown in FIG. 16 is known where a DC power source 103 is connected to a corona wire 102 of a corona charging device (scorotron) 100 having a grid 101, and at the same time prescribed control bias voltage is applied to the grid 101 and charging voltage of an image carrier 104 is controlled to level of the control bias voltage of the grid 101.
Also as shown in FIG. 17, such a two-color image forming apparatus is provided that charging of the image forming cycle of the first color is carried out by a DC corona charging device 105, and charging of the image forming cycle of the second color is carried out by an AC corona charging device 106 (e.g., JPB No. 36230/1991).
In such a color image forming apparatus, however, a toner layer exists on the image carrier in the image forming cycle of the second color or later, but if the charging is carried out in the state that the toner layer exists, the absolute value of the surface potential of the toner layer rises significantly due to the charge of the toner layer itself and the absolute value of the surface potential of the image carrier itself holding the toner layer thereon is relatively decreased.
Therefore, for example, in a type of adopting the reversal development as developing system, even if the exposure is carried out in the image forming cycle of the second color or later, technical problem is found in that it becomes difficult to secure sufficiently the difference between the surface potential of the image carrier and the potential of the exposed portion, that is, the latent image contrast potential, and corresponding to this, the upper layer toner is not developed sufficiently, which results in the cause of the color variation.
As means for solving such a technical problem, for example, technology is already provided where two DC corona charging devices are arranged continuously and the potential allocated to the image carrier is made large and the latent image contrast potential is raised (e.g., JPA No. 209274/1991).
In this connection, as shown in FIG. 18, technology is already provided where, for example, a DC power source 113 in the same polarity as the charged polarity is connected to a corona wire 112 of a first corona charging device 111, and on the other hand, a DC power source 116 in the opposite polarity to the charged polarity is connected to a corona wire 115 of a second corona charging device 114, and the absolute value of the surface potential of the image carrier 104 is charged to the charging potential required for the development or more by the first corona charging device 111 and then set to the charging potential previously set by supplying the reverse charge by the second corona charging device 114.
According to this type, charge injected by the first corona charging device 111 is more than that in a type using single charging device, and charge injected by the second corona charging device 114 mainly neutralizes the charge on the surface of the toner layer and reverses it.
Therefore according to the electric field produced by the toner layer itself, the charge injected by the first corona charging device 111 is apt to distribute much in the vicinity of the surface of the image carrier 104 and the potential allocated to the image carrier 104 is secured large, and the charge on the surface of the toner layer becomes that reversed to the charge on the surface of the image carrier 104 and corresponding to this, the toner on the surface of the toner layer is attracted to the side of the image carrier 104 electrostatically.
Consequently even if the thickness of the toner layer becomes large, the potential of the image carrier can be secured sufficiently large and corresponding to this, decrease of the contrast of the latent image potential in the color superposing can be suppressed.
However, when the second toner image is produced in superposing on the first toner image, for example, in the case of color character or color image, in the exposure process of the second image forming cycle, disadvantage is produced so that the first toner image is scattered in the surrounding of the second exposure region (toner scattering phenomenon) and sharpness of the image is deteriorated, and in the development process of the second image forming cycle, the first toner image is transferred into the second developing device (reversal development phenomenon) thereby technical problems such as the optical density deterioration, the color variation and the color mixing are produced.
These are caused by that the first toner image is scattered to the environment of the exposure region with high surface potential due to the discharge of the reverse charge by the second corona charging device and subjected to the reversal development due to bias of the developing roll of the second developing device.